Electrical resistance material and method of making same



A'Ug- 30, 1960 T. M. PLACE, SR., TAL 2,950,996

ELECTRICAL RESISTANCE MATERIAL AND METHOD CE MAKING SAME Filed Deo. 5, 1957 Hea o Dry m70 Deco/71,0056 Res/'na fe F/Ym on Base United States Patent O ELECTRICAL RESISTANCE MATERIAL AND METHOD F MAKING SAME Thomas M. Piace, Sr., Newport Beach, and Thomas M. Place, Jr., Costa Mesa, Calif., assignors to Beckman Instruments, Inc., a corporation of California Filed Dec. 5, 1957, Ser. No. 700,746 21 Claims. (Cl. 117-227) This invention relates to electrical resistance material suitable for use in ixed resistors, variable resistors, potentiorneters `and the like `and to methods of making same. The invention is particularly related to a ceramic type of resistance material which may be applied in a relatively thick lm to a nonconductive refractory base. This invention is an improvement on `the invention of our copending applications entitled, Electrical Resistors and Method of Making the Same, Serial No. 439,650, iiled June 28, 1954 (now abandoned), and Electrical Resistance Element and Method of Making Same, Serial No.

646,888, tiled March 18, 1957, both of which are assigned to the `assignee of the present invention.

In a preferred embodiment, the resistance element of our copending applications is a layer or ilm or resistance material comprising a heterogeneous mixture of ceramic glass and conducting metals iixed to a nonconducting base with the resistivity and temperature coeiiicient off resistance of the material dependent upon the proportions of the ceramic lglass and conducting metal and upon the particular metal or combination of metals utilized in the resistance material. The layer or ilm is formed by heating the mixture to a temperature exceeding the melting point of the ceramic glass but below the melting. point of the metals to create a smooth, continuous glassy phase. The particular metal or combination of metals and the relative proportion of metal and ceramic Iglass also affect the contact resistance and electrical noise. Of course, consistently low contact resistance and low noise value are desirable when the resistance material is used with sliding contacts or wipers such as is the case in a potentiometer. l

However, it is sometimes found that the particular combination of metal and ceramic glass giving the desired resistivity and temperature coefficient of resistance has an undesirably high contact resistance and noise value. Accordingly, it is an object of the invention to produce a ceramic resistance material in Which the contact resistance and noise may be controlled at relatively low values. Another object of the invention is to provide additional materials which can be added in small amounts to the conducting metal-ceramic glass mixture, which additional materials will have marked eiiects on the resistivity and temperature coeiiicient of resistance without adversely affecting the contact resistance or noise characteristic. It has Ibeen found that in a speciiic composition of metals and ceramic glass having a desired contact resistance and noise value, theA temperature coecient of resistance and the ohmic resistance can be p 2,950,996 ce Patented Aug. 30, 1960 controlled by the addition of small percentages of oxide semiconductors.

The term oxide semiconductor as used herein means a binary or complex oxidic compound of the polar type in which electropositive and electronegative constituents can be clearly distinguished and may be considered as a solid with an ionic lattice.' At least some of the metal ions in the oxidic compound are derived from an element or elements belonging to the transition series, i.e. elements ranging from titanium to zinc in the periodic table. Such oxide semiconductors are in extensive use for various purposes as sintered bodies produced by ceramic techniques. Solid phase reactions result from the high temperature sintering, producing ions of the same element with different valences at equivalent lattice points in the ionic lattice. Specific examples of oxide semiconductors are set forth hereinafter and include,

- for example, copper stannate, copper antimonate, zinc stannate, Zinc antimonate.

Accordingly, it is an object of the invention to provide a resistance material comprising a ceramic or silicate glass, having finely divided, colloidal or molecular-sized particles of metal and oxide semiconductors dispersed throughout the glass with the glass being the predominant portion of the resistance material and the metal being a relatively small portion and the oxide semiconductor being a fraction of the metal portion. Another object of the invention is to produce such a resistance material by the dispersion of colloidal or molecular sized particles of the metal in the ceramic glass which is melted into a smooth, continuous phase at a temperature below the melting point of the metals and oxide semiconductors employed. A further object of the invention is to provide such a resistance material in which the metal comprises not more than sixteen percent by Weight of the resistance material and the oxide semiconductor comprises not more than three percent by weight. A further object of the invention is to provide a resistance element in which such a resistance material is formed in relatively thick ilrns, such lms being in the order of .0005 to .003 inch thick.

It is a further object of the invention to provide methods for manufacturing the resistance material of the invention and resistance elements employing the resistance material.'

The resistance material of the invention comprises the fused mixture of a nonconducting ceramic or silicate glass, metal, and oxide semiconductor with trace amounts of certain other materials added when desired. The glass must not absorb moisture and must be resistant to high humidity and fungus and must fuse to a smooth surface, continuous glassy phase on heating to a temperature below the melting point of the meta-l or metals and oxide semiconductor or semiconductors mixed therewith. The particular composition of the glass utilizedis not critical to the practice of the invention and various changes in the composition of the glasses can be made to alter the fusion temperature, coetiicient of thermal expansion, fluidity, solubility, etc., by one familiar with the ceramic arts to provide a particular desired characteristic. Ceramic glasses suitable for this purpose are lead borosilicate glasses. The composition of a glass which has been used in the practice of the invention is given below as illustrative, but is in no way intended to be restrictive to the composition used in the resistance material.

While the glass may be produced by any conventional process, it is preferred that it be as homogeneous as possible. One method of making a glass includes thoroughly mixing a batch of rauw materials together while dry, melting the batch in ceramic crucibles to produce a clear, fluid glass, quenching the molten glass by pouring into cold water, and crushing and then grinding the resulting shattered glass in methanol or similar liquid to a very fine powder with all particles less than about 325 mesh in size.

The metal or metals used in the mixture must be nonreactive and nonoxidizable. The term nonreactive7 means that the metal will not react substantially with the other components of the mixture either at room temperature or `at the elevated temperatures required to produce the continrous, glassy resistance element. The term nonoxidizable means that the metal does not oxidize substantially in a normal atmosphere at such elevated temperatures. Such metals are commonly referred to as noble metals and for the purposes of this specification include gold, silver, palladium, platinum, rhodium, and iridium. However, this is not intended as an exclusive listing since other metals are known to have similar properties and may be used in the practice of the invention and are intended to be included in the class of noble metals. Furthermore, mixtures and alloys of these noble metals can also be used.

The oxide semiconductors are added to the glass-noble metal mixture to provide close control ofthe electrical characteristics of the resultant resistance material. Such oxide semiconductors show no measurable conductivity when dispersed in a glass inthe absence of the noble metal. The phenomena occurring in the glass-nobleoxide semiconductor mixture'of the invention are unexpected and not yet fully understood. However, it appears that the oxide semiconductors are dispersed as colloidal particles between or surrounding the metal particles, affecting the passage of electrons between these metal particles within the glass. The preferred oxide semiconductors for use in the resistance material of the invention include metal stannates and metal antimonates and mixtures thereof, but metal titanates, manganates, vanadates, arsenates, molybdates, and mixtures thereof or with metal stannates or antimonates can also be used. Examples of such oxide semiconductors particularly useful in the invention are the following metal stannates and antimonates, used separately or together in the composition; namely, copper stannate, zinc stannate, ferrous stannate, ferric stannate, cadmium stannate and manganese stannate, and copper antimonate, zinc antimonate, ferrous antimonate, ferric antimonate, cadmium antimonate, lead antimonate and manganese antimonate. The addition of such materials to the glass-noble metal mixture varies the electrical characteristics of the material. Specifically,

` the addition of ferric stannatel raises the ohmic resistance and moves theV temperature coefficient of'resistance to positive values. v Zinc stannateraises the ohmic resistance sistance, temperature coeicient, etc.

with llittle change in temperature coefficient of resistance. Copper stannate raises the ohmic resistance and changes the coeliicient of resistance toward negative values. Copper antimonate increases the ohmic resistance and produces high positive temperature coefficient of resistivity. loint use of both metal stannate and metal antimonates is particularly useful in some compositions.

The glass-noble metal-oxide semiconductor mixture used in forming the resistance material of the invention is predominantly glass with a relatively small amount of noble metal and not more than three percent by weight of oxide semiconductors. Since total percentages of oxide semiconductors less than about one percent in a given composition have little practical effect upon its electrical characteristics, the preferred range of oxide semiconductors is between about 1 and 3 percent by weight. When referring to percentages of mixture by weight throughout the specification, we are referring to the proportions in the finished resistance material after firing, without considering the volatile carriers used in various stages of preparing the mixture. It is preferable in the preparation of the resistance materials of the invention to utilize copper stannate or copper stannate mixed with copper antimonate and it is also preferable to have the oxide semiconductors, of whatever form, comprises about 41-69 percent by weight of the noble metal.

The particular proportion utilized in a specific resistance element will depend upon the desired value of re- I-lowever, the range of proportions will be ceramic glass about d4-98 percent by weight and noble metal about 1-15 percent by weight. The preferred range within which most resistance elements of the invention fall is 91-97 percent by weight of glass and 2-8 percent by weight of metal.

t has been found that introduction of trace amounts of one or more refractory metal oxides, commonly known as opacifiers, to the glass-noble metal oxide semiconductor mixture reduces the contact resistance between moving contacts and the surface of the solidified mixture. When added in small percentages to ceramic glasses, opacifiers having the property of being uniformly dispersed as colloidal particles or flocs when the glass is melted and subsequently cooled, thereby producing a more uniform dispersion of metal particles throughout the resistance material when utilized in the invention. Examples of such opaciers are tin oxide, antimony oxide, zirconia, molybdenum oxide and chromium oxide. Accordingly, although not essential to the performance of the invention, it is preferred in the practice of the invention to include fractional percentages and not more than 1/2 per- 4cent by weight of one or more of the opaciflers in the mixture of glass, noble metals and oxide semiconductors, particularly when the resistance material is to be used in a potentiometer.

lt has been found that the inclusion of a fractional percentage of low melting temperature ceramic flux, such as bismuth oxide, molybdenum oxide or vanadium oxide in the glass-noble metal oxide semiconductor mixture produces adhesion of the metal particles to glasspartioles at temperatures below the softening point of the glass and tends to prevent agglomeration of the metal particles as the firing temperature increases. Accordingly, although not essential to the performance of the invention, it is preferred in the practice of the invention to include a fractional percentage or trace amount and not more than 1 percent by weight of the low melting temperature ceramic flux in the glass-noble metal oxide semiconductor mixture.

A wide range of compositions of glass, noble metals and oxide semiconductors may be used in making the resistance material of the invention for producing a wide range of electrical characteristics. By way of example and not of limitation, the following percentage compositions are set out as illustrative of the range of mixtures covered by the invention.

Mixture No. 11 12 13 14 15 16 17 18 19 20 9 Bismnth Oxide .40 32 .25 .22 .25 .25 27 27 16 Tin Oxide 08 .06 .05 .05 O5 .O5 05 Chrome Oxide 08 .06 .05 .05 .05 05 05 Copper Stanuate 2. 35 2. 00 1. 65 1.88 2. 48 1. 98 1. 70 1.23 1. 49 Copper Antimonate- 27 22 19 .81 93 90 Resistivity, Ohms! Square 55 415 557 20 4, 180 2, 567 1, 294 38, 633 41 7, 500 Temperature Coefficient, p.p.m./ F 0 0 +44 +213 112 +191 +22 -221 +1.38 -12. 4

A flow diagram of the invention is shown in the single gure of the drawing.

In a preferred method of preparing the resistance material of the invention, the noble metals or mixtures or alloys thereof are introduced as soluble metal compounds which will be decomposed by heat, leaving colloidal or molecular-sized particles. Metal organic compounds such as metal resinates, which are commercially available dissolved in essential oils, are suitable for this purpose. Since commercial metal resinate solutions usually contain fractional percentages of tin oxide, chromium oxide and bismuth oxide, etc. as impurities, these oxides serve as the opaciiiers and uxes previously mentioned.

The oxide semiconductor materials are ground in methanol, dried, crushed to a powder and calcined at approximately 2000 F. for at least two hours. The calcines are then ball milled with methanol to pass 325 mesh. The ceramic glass powdered as described above, the metal resinate solution and the oxide semiconductor are separately weighed to determine the desired proportions and then ground together thoroughly so that each grain of glass and calcine is thoroughly wet with the metal resinate solution.

This fluid mixture is placed in an open porcelain container on an electric hot plate or the like, preferably with a stirrer arranged to keep the mixture in continual agitation. Low heat is applied to evaporate the solvents with the heat being gradually increased to decompose the resinates leaving colloidal particles covering each glass grain. This mixture is crushed to a ne powder and calcined at about 800 F. to remove any carbon which might remain from the solvents or resinates. '111e resulting powder is ground in methanol to pass 325 mesh and may be stored indefinitely without change or deterioration.

When it is desired to produce a resistance element, the resistance material may be applied to a suitable refractory base. The base material may be of any suitable electrically nonconducting material which will withstand the elevated temperatures used in ring the resistance material. Various ceramic products are suitable for this use, the necessary characteristics being a smooth, fine textured surface which is vitriiied and practically impervious to moisture and other liquids. Steatite, fosterite, sintered or fused aluminas and zircon porcelains are examples of preferred materials for forming the base. The resistance material may be applied to the base by any suitable means such as brushing, spraying, stenciling or silk screening. The prepared powdered resistance material can be mixed or milled with a suitable organic carrier vehicle commonly used in the paint or silk screen arts. The proportioning and mixing of such preparations for any particular mode of application are well known.

After the film of resistance material has been applied to the base, it is permitted to dry in circulating warm air until the Volatile solvents have evaporated. The carrier or vehicle generally used contains suflicient organic binder that when dried, the surface of the resistance material is sufficiently hard to withstand normal handling without marring or blemishing.

Following the drying step, the resistance material is fired to fuse the glass into -a continuous glassy phase. Conventional ceramic kilns are suitable for this operation, one utilizing electrical heat being preferable because of its cleaner atmosphere. The temperature to which the material is tired must not be so low as to result in failure to achieve the desired continuous glassy phase with a hard smooth surface and must not be so high as to produce undesired bubbles or blisters and agglomeration of the metal particles. The tiring temperature depends upon the particular glass utilized and upon the particular kiln and cannot always be determined in advance, since the heating characteristics ofindividual kilns vary. The time and temperature cycle of the tiring step is not otherwise critical and one skilled in this ceramic art can devise a number of suitable firing schedules.

It is preferred to re the material on a two-phase tiring cycle and the following is illustrative of a suitable tiring schedule. The base with the tilm of resistance material is placed in the kiln and the temperature is increased to l000 F. at a rate of approximately 400 F. per hour. The temperature is then held at 1000 F, for about thirty minutes to assure the removal of all volatile and organic materials from the mixture and, also, to insure the uniform distribution of heat throughout the base and iilm of material before the glass starts to fuse. Then the temperature of the kiln is raised to 1490 F. at a rate of about 200 F. per hour. The temperature is maintained at the l490 point for thirty minutes to insure uniform heat distribution and at the end of this period the kiln is allowed to cool to room temperature by normal radiation. The ring procedure is conducted in a normal oxidizing atmosphere. The heating cycle and temperature may be varied over a wide range as required for different glasses and diEerent kilns to complete the desired reaction.

When cool, the, lm of resistance material is permanently attached to the base in the form of a smooth, black, glossy lm retaining the exact size and shape in which it had been applied to the base.

The iilms of resistance material can be applied to a base of any desired configuration depending upon the particular application. The films of resistance material will range from a fraction to a few thousandths of an inch in thickness with the preferred range being about .0005- .003 inch. The majority of the resistance elements being produced according to the invention at the present time are in the order of one thousandth of an inch thick. Since the resistance layer has a substantial thickness as compared to the sputtered and evaporated metallic iilm resistors, thickness control is much less critical in the resistance material of the invention.

Electrodes may be applied as required at the ends or intermediate points of the film of resistance material permitting the attachment of leads or other conductors by soldering or other means. The electrodes may be formed of any of the conventional silver or other metal pastes, the paste being applied at the desired location, following which the unit is red to convert the paste to an electrical conductor.

Although exemplary embodiments of the invention have 1. A resistance material yfor use in'a'resistor or the like, which comprises about 84-98 percent by Weight of ceramic glass and about I-lS'percent byrweight of at -least one of the noble metals and about l-3 percent-by Weight of at least one of the complex metal-oxide semiconductors.

oxide semiconductors, the metals and semiconductors being in tinely divided form and dispersed throughout Y the glass in electrically conductive relationship.V

ll. A resistance element comprising a coating of resistance material fired ona high-temporatore-resistant,

electrically nonconductive base, Ywhich coating'comprises about Sli-98 percent by Weight ot solidified ceramioglass and aboutY 1-,1/5 percent by Weight of'at least one of the noble metals and about 1 3 percent byu/eight of complex metal-oxideY semiconductors, the metals and oxide semiconductors being in tinely divided form and dispersed 2. A resistance material for use in a resistor or the like, which comprises about 84-98 percent by WeightV of ceramic glass and about l-15 percent by Weight of at least one of the noble metals and about 1 3 percent by Weight of oxide semiconductors selected from the group consisting of copper stannate, zinc stannate, ferrous stannate, ferrie stannate, cadmium stannate, and manganese stannate.

3. A resistance material for use in a resistor or the like, which comprises about 84-98 percent by Weight of ceramic glass and about l-l5 percent by Weight of at least one of the noble metals and about l-3` percent by Weight of oxide semiconductors selected fromthe group consisting of copper antimonate, Zinc antimonate, ferrous antimonate, ferric antimonate, cadmium antimonate, lead antimonate, and manganese antimonate.

4. A resistance material for use in a resistor'or the like, which comprises about 84-98 percent by Weight of ceramic glass and about lpercent by weight of at least one of the noble metals and about l-3 percent by weight of a combination of oxide semiconductors, one component of the combination being selected from the group consisting of copper stannate, zinc stannate, ferrous stannate, ferrie stannate, cadmium stannate, and manganese stannate, and the other component of the combination being selected from the group consisting of copper antimonate, zinc antimonate, ferrous antimonate, ferrie antimonate, cadmium antimonate, lead antimonate, and manganese antimonate.

5. A resistance material for use in a resistor or the like, which comprises about 84-98 percent by Weight of ceramic glass and about l-lS percent by Weight of at least one of the noble metals and about 1-3 percent by Weight of copper stannate.

6. A resistance material for use in a resistor or the like, which comprises about 84-98 percent by Weight of ceramic glass and about 1-15 percent by weight of at least one of the noble metals and about 1-3 percent by r Weight of a mixture of copper stannate and copper antimonate.

7. A resistance material for use in a resistor or the like, which comprises about 84-98 percent by Weight of ceramic glass and about l-l5 percent by Weight of at least one of the noble metals and about l-3 percent by weight of zinc stannate.

8. A resistance material for use in a resistor or the like, which comprises about 84-98 percent by Weight of ceramic glass and about l-l5 percent by Weight and about 1-3 percent by weight of a mixture of zinc stannate and Zinc antimonate.

9. A resistance material for use in a resistor or the like, which comprises about 84-98 percent by weight of ceramic glass and about l-l5 percent by Weight of at least one of the noble metals and a quantity of complex metal-oxide semi-conductors in the amount of 41-69 percent by weight of the quantity of noble metals, the metals and oxide semiconductors being in finely divided form and dispersed throughout the ceramic glass in electrically conductive relationship.

l0. A resistance material for use in a resistor or the like, comprising 91-97 percent by Weight of ceramic glass and 2-8 percent by Weight of at least one ofthe noble metals and 1-3 percent by Weight of complex metalthroughout the Vsolidiiied ceramic glass in electrically conductive relationship with the coating being about .0005- .003 inch in thickness. e

12. A resistance material for use in a resistor or the like, which comprises about Sli-98 percent Vby- Weight of ceramic glass and about l-lS percent by Weight of at least one of the noble metals and about 1 3 percent by f Weight of complex metal-oxide semiconductors, the metals and oxide semiconductors being in iinely divided form and dispersed throughout the glass in electrically conductive relationship, and less than l percent by Weight of a low melting temperature ceramic ilux dispersed ,throughout the glass.

13. A resistance material for use ina resistor or the like, Which-comprises about 8411-68 percent by Weight of ceramic glass and about l-ld percent by Weight of vat least one of the noble metals and about 1,-3 percent by Weight of complex metal-oxide semiconductors, the metals and semiconductors being in finely divided form and dispersed throughout the-glass in electrically conductive relationship, and less than 1/2 percent by Weight of opaciiier dispersed throughout the glass.

14. A resistance material for use in a resistor or the like, which comprises about 84-98 percent by Weight of ceramic glass and about l-l5 percent by Weight of at least one of the noble metals and about 1 3 percent by Weight of complex metal oxidesemiconductors, the metals and oxide semiconductors being in finely divided form and dispsersed throughout the glass in electrically conductive relationship, and less than l percent by Weight of low melting temperature ceramic ilux and less than 1/2 percent by weight oi opacier, said iiux and opacirier being dispersed throughout the glass.

l5. A resistance element comprising a iilm of colloidal particles of noble metal and complex metal-oxide semiconductors dispersed throughout a continuous phase of ceramic glass, the melting Vtemperature of said glass being lower than the melting temperatureof said metals and said oxide semiconductors, said ilm being coated on a high-temperature-resistant, electrically nonconductive base, said metal comprising about 1-15 percent by Weight of the total of said iilm, saidoxide semiconductors comprising about l-3 percent by Weight of the total of said lm. Y

i6. r[he method of' making an electrical resistance, element which comprises: forming a viscous mixture containing a volatile liquid carrier, powder-like particles oi glass, at least one noble metal and at least one complex metal-oxide semiconductor having Vmelting points above that of said glass, the solid elements oi said mixture including about d-'percent byweight of said glass, about l-lS percent by weight of such metal-and not more than about 3 percentby Weight of such oxide semiconductor; forming at least a portion of said mixture into a iilm on a supporting structure; heating said nlm to an intermediate temperature to remove the volatiles and organic material therein; Vadditionally heating said iilm to a pretetermined temperature exceeding the melting point of said glass but less than the melting points of such metal and oxide semiconductors', and cooling said lilm to asolid state.` Y s l7. ln a method of manufacturing anelectrical resistance element, the steps of: mixing a volatile liquid with ,finely ground glass, `at least one noble metal, and at least one complex metal-oxide semiconductor to produce a viscous mixture, the solid elements of said mixture including about 84-98 percent by weight of said glass, about lpercent by weight of such metal, and not more than about 3 percent by weight of such oxide semiconductor; applying the viscous mixture to a hightemperature-resistant, electrically nonconducting base in a lm having a lthickness of between about .0005 and .003 inch; heating the base and film in an oxidizing atmosphere to a temperature below the melting point of the glass to remove the volatiles and organic material therein; additionally heating the base and lm to a predetermined temperature less than the melting points of the metal and the oxide semiconductor to produce a continuous glassy phase having a smooth surface with the metal uniformly dispersed therethrough; and cooling the layer to a hardened state.

18. In a method of manufacturing van electrical resistance element, the steps of: forming a uniform mixture containing Ia finely ground glass, a solution of at least one noble metal-organic compound, and at least one complex metal-oxide semiconductor; applying a film of the mixture to a high-temperature-resistant nonconducting base; heating the base and film to an intermediate temperature to reduce the metal-organic compound and to remove the volatiles and organic material therein; additionally heating the base and lm to a predetermined temperature less than the melting points of such noble metal and oxide semiconductor to produce a continuous glassy phase having a smooth surface with the metal uniformly dispersed therethrough; and cooling the layer to a hardened state.

19. A method of manufacturing an electrical resistance element including the steps of: heating a mixture of finely ground glass, at least one noble metal, at least one complex metal-oxide semiconductor, and a liquid vehicle to remove the volatile `and organic material therein producing a dry mixture; grinding the dry mixture to a powder; heating the ground powder to produce calcination; grinding the calcined product to a fine powder; mixing the fine powder with a volatile liquid to form a viscous mixture; applying a film of the viscous mixture to a. high-temperature-resistant, electrically nonconducting base; heating the base and film in an oxidizing atmosphere to a temperature below the melting point of the glass; additionally heating the base and film to 10 a predetermined temperature exceeding the melting point of the glass but less than the melting points of such metal and such oxide semiconductor; and cooling the layer to a hardened state.

20. A method of manufacturing an electrical resistance element including the steps of: forming a uniform mixture containing a finely ground glass, a solution of at least one noble metal resinate, and a solution of at least one complex metal-oxide semiconductor resinate; heating the mixture to reduce the resinates and to remove the volatiles and organic materials therein producing -a dry mixture; grinding the dry mixture to a powder; heating the ground powder to produce calcination; grinding the calcined products to a fine powder; mixing the fine powder with a volatile liquid to form a viscouse mixture; applying a film of the viscous mixture to a hightemperature-resistant, electrically nonconducting base; heating the base and film in an oxidizing atmosphere to a temperature below the melting point of the glass; additionally heating the base and film to a predetermined temperature exceeding the melting point of the glass but less than the melting point of such metal and such oxide semiconductor; and cooling .the film to a hardened state.

21. A method of varying the electrical characteristics of an electrical resistance element formed of a fused mixture of about 84-98 percent by Weight of glass and about 1-15 percent by weight of at least one noble metal, which method includes the steps of: adding about 1-3 percent by weight of complex metal-oxide semiconductors to a mixture of said glass and noble metal while the glass is in finely divided form; applying a film of the resultant mixture to a high-temperature-resistant, electrically nonconductive base; and heating the resulting mixture to a temperature less than the melting temperatures of the metal and metal oxide semiconductor to produce `a continuous glassy phase having a smooth surface with the metal and metal oxide semiconductor uniformly dispersed therethrough.

References Cited in the le of this patent UNITED STATES PATENTS 2,461,878l Christensen et al. Feb. 15, 1949 2,552,626 Fisher et al. May 15, 1951 2,588,920 Green Mar. 11, 1952 2,786,819 Smith et al. Mar. 26, 1957 2,855,491 Navias Oct. 7, 1958 

16. THE METHOD OF MAKING AN ELECTRICAL RESISTANCE ELEMENT WHICH COMPRISES: FORMING A VISCOUS MIXTURE CONTAINING A VOLATILE LIQUID CARRIER, POWER-LIKE PARTICLES OF GLASS, AT LEAST ONE NOBLE METAL AND AT LEAST ONE COMPLEX METAL-OXIDE SEMICONDUCTOR HAVING MELTING POINTS ABOVE THAT OF SAID GLASS, THE SOLID ELEMENTS OF SAID MIXTURE INCLUDING ABOUT 84-98 PERCENT BY WEIGHT OF SAID GLASS, ABOUT 1-15 PERCENT BY WEIGHT OF SUCH METAL, AND NOT MORE THAN ABOUT 3 PERCENT BY WEIGHT OF SUCH OXIDE SEMICONDUCTOR, FORMING AT LEAST A PORTION OF SAID MIXTURE INTO A FILM ON A SUPPORTING STRUCTURE, HEATING SAID FILM TO AN INTERMEDIATE TEMPERATURE TO REMOVE THE VOLATILES AND ORGANIC MATERIAL THEREIN, ADDITIONALLY HEATING SAID FILM TO A PREDETERMINED TEMPERATURE EXCEEDING THE MELTING POINT OF SAID GLASS BUT LESS THAN THE MELTING POINTS OF SUCH METAL AND OXIDE SEMICONDUCTORS, AND COOLING SAID FILM TO A SOLID STATE. 